A Memcache compatible plugin for the Apache Traffic Server

SECURITY: this plugin exposes the shared ATS cache (read/write/delete) with no
authentication and no ip_allow ACL on its accept path. It binds to loopback
(127.0.0.1) by default; only bind a non-loopback address behind a firewall that
restricts access to trusted clients. See doc/admin-guide/plugins/memcache.en.rst.
